DESCRIPTION This proposal describes an approach to the regulation of protein-protein interactions via a small ligand. Protein-protein interactions are very important in many processes in the cell, including transcription, translation, signal transduction and cell growth. The control of these processes is a powerful tool and is of interest to many industrial and academic groups. Our approach involves mutagenizing one of the proteins in a known protein-protein interaction to abolish binding. The protein pair will then be screened in the presence of a library of ligands for the reemergence of protein-protein binding. The binding between the human growth hormone and the human growth hormone receptor will be used as a model to test this approach. The substitution of alanine for tryptophan 104 of the human growth hormone receptor has been shown to abolish its binding to the human growth hormone. The library of ligands will be based on an indole scaffold and will be screened using the yeast two-hybrid system for the re-establishment of protein-protein binding. To further increase shape complementarity of the protein for the ligand, DNA shuffling of the human growth hormone receptor will be carried out. If successful this methodology could be expanded to other areas such as the regulation of binding between an insulin variant to the insulin receptor.